Yersinia pestis is the causative agent of plague, a severe acute illness that is relatively rare in the United States today. However, there is concern that Y. pestis will be utilized as a bioweapon due to its ability to cause high mortality in an unprotected population. Since currently there are no available human vaccines that effectively prevent Y. pestis infections, there is a critical unmet need for the identification of protective immunogens against this pathogen. Preliminary studies and investigations have led us to hypothesize that vaccination with membrane proteins derived from Y. pestis grown under conditions that mimic in vivo growth will protect against subsequent infection and mortality. To test this hypothesis, and work towards our overall goal of developing safe and effective vaccines against Y. pestis infection in humans, we hereby propose Phase I studies with three specific aims: In Aim 1, we will compare the protective efficacy of membrane proteins derived from Y. pestis grown at 37[unreadable]C in iron-limiting and iron-rich conditions in mouse models of bubonic and pneumonic plague; in Aim 2, we will separate the proteins within the composition providing the best protection in Aim 1, test the resulting fractions for reactivity to serum obtained from immunized mice, and identify proteins within sero-reactive fractions; and in Aim 3, we will combine sero-reactive fractions into vaccines and test their efficacy in mouse models to determine which proteins contribute to protection against plague. Taken together, we believe that the proposed investigations will provide key information about specific antigens that may be used for the development of an effective vaccine for the prevention of Y. pestis infections. Yersinia pestis is the causative agent of plague, a serious and potentially fatal infection that is relatively rare in the United States today. However, the intentional use of Y. pestis as a bioweapon would inflict a high mortality rate in an unprotected population, and there are currently no available vaccines for the effective prevention of plague. In the proposed studies, we plan to identify protective antigens as an initial step towards our overall strategy for developing vaccines for the prevention of human plague. [unreadable] [unreadable] [unreadable]